The present invention relates to a method for rotation speed control of a rotary element in the drive line of a vehicle.
The term “rotary element” means a shaft, such as a longitudinal drive shaft (for example propeller shaft) or a transverse drive shaft (that is to say wheel axle) or other power transmission element forming part of the drive line and adapted for rotation.
The term “drive line” means the entire power transmission system from the engine of the vehicle to the ground engagement elements. The drive line therefore includes clutch, gearbox (and any transfer gearbox present), propeller shaft (or propeller shafts), transverse drive shafts etc. Hydraulic, electric and other drive systems are also included within the term drive line.
The term “ground engagement elements” includes wheels, caterpillar tracks etc.
The invention can be applied to wheel-borne vehicles, track-borne vehicles and vehicles running on rails. Primarily wheel-borne vehicles are intended. The invention can also be applied to passenger cars, trucks, buses and other road vehicles but is primarily intended for cross-country vehicles, such as four-wheel drive passenger cars, and working vehicles, such as frame-steered dumpers, wheel loaders, excavators etc. The invention is particularly applicable in vehicles with a plurality of driven axles and will below be described for a frame-steered dumper for the purpose of exemplification.
A fundamental problem for all vehicles with drive at a number of ground contact points is how the driving power is distributed. It is desirable to control the rotation speeds of the wheels so that the slip in the longitudinal direction is the same at all ground contact points because this results in excessive slip at individual ground contact points being prevented. Slip is the standardized difference between the speed of the wheel at the ground contact point and the speed of the ground at the same point.
One way of bringing about the desired identity of longitudinal slipping would be to connect the drive of all the wheels mechanically. However, this would not work during cornering. During cornering, the ground moves at different speed at the various ground contact points. The ground under the outer wheels moves at higher speed than the ground under the inner wheels because the outer wheels have a greater distance to cover in the same time as the inner wheels. During cornering, the ground under the front wheels also moves at higher speed than the ground under the rear wheels.
The problem of distributing tractive power in an effective way during cornering as well is conventionally solved by dividing the torque in a given, fixed ratio with the aid of a differential. The rotation speed is then controlled by the speed of the ground at the various ground contact points and by the slip. However, the slip cannot be controlled. If the product of vertical load and ground friction does not correspond to the torque ratio in the differential, the slip can increase unlimitedly, the wheels slip and the total tractive power transmitted is limited by the slipping ground contact.
The problem of uncontrolled slip is usually reduced by various measures for braking the slip, for example by using what is known as a differential lock. The differential lock conventionally comprises a claw coupling which locks the differential mechanically. The disadvantage of differential locks is that the speed difference during cornering is offset as slip at the ground contact points concerned. This results in great constrained torques which shorten the life of the drive line, give rise to losses and cause great tire wear.
WO03/006846 describes a large number of different drive line solutions which afford opportunities for remedying the abovementioned problems during cornering.
It is desirable to provide a method for controlling the rotation speed of a rotary element in the drive line of a vehicle in a way which results in a longer life of the drive line and/or lower losses in the form of fuel consumption and/or tire wear.
A method according to an aspect of the present invention is provided for controlling rotation speed of at least one rotary element in the drive line of a vehicle, at least one operating parameter being detected repeatedly, which operating parameter corresponds to an actual value of a torque in the drive line which is delivered to the rotary element, a desired value of a torque to the rotary element being determined on the basis of friction against the ground of at least one of the ground engagement elements of the vehicle, which ground engagement element is driven via the rotary element, and the rotation speed of the rotary element being controlled so that the actual value moves toward the desired value.
In this way, it is possible actively to control the torque which is distributed from the drive source of the drive line to a specific ground engagement element or drive shaft as required. It is therefore possible to vary the torque distribution to different ground engagement elements or drive shafts depending on prevailing operating conditions. The term “operating condition” is to be understood in a broad sense here and can include, for example, operating parameters detected in the vehicle, current driving situation/task, geographical position, environment/weather etc.
According to a preferred embodiment, a vertical load from the vehicle toward the ground at the ground contact point of the ground engagement element is determined on the basis of the detected operating parameter, and the vertical load determined is used for calculating the desired value of the torque. To be precise, the vertical load provides an indication of the friction against the ground. According to a development, the speed of the vehicle is detected and is used for calculating the desired value of the torque.
The driving torque is preferably controlled so that the coefficients of friction at the ground contact points are essentially the same.
According to another preferred embodiment, the actual value of the torque is detected by a regulating motor which is operationally coupled to the rotary element. The regulating motor is preferably adapted to supply a calculated rotation speed increase to the rotary element when necessary.
According to another preferred embodiment, a regulating unit controls the rotation speed of the rotary element so that the rotation speed which is supplied to two different wheel engagement elements is varied. The regulating unit preferably comprises the regulating motor according to the preceding embodiment.
According to another preferred embodiment, the rotation speed of the rotary element is controlled on the basis of the torque determination, and the slip at the ground contact point of the ground engagement element is allowed to vary.
According to another preferred embodiment, the rotation speed of the rotary element is controlled on the basis of a determined torque distribution between at least two of the ground engagement elements of the vehicle. By dividing the drive line into a number of branches from the drive source to the ground engagement elements, it is possible to determine a distribution of the torque between the different branches and to control the rotation speed of the rotary element(s) correspondingly. The torque distribution is suitably determined according to prevailing operating conditions.
According to another preferred embodiment, rotation speed control is carried out between a front drive shaft and a rear drive shaft in the vehicle so that the rotation speed of the front drive shaft is increased in relation to the rotation speed of the rear drive shaft during cornering. The tractive power on the front drive shaft in relation to the vertical load on the front drive shaft is preferably controlled to a given ratio relative to the tractive power on the rear drive shaft in relation to the vertical load on the rear drive shaft.
According to a preferred example, the rotation speed is controlled so that the ratio between the tractive power and the wheel pressure is essentially the same at the ground contact points.
According to a further embodiment, which is an alternative or complement to the preceding embodiment, rotation speed control is carried out between a right and a left ground engagement element so that the rotation speed of the ground engagement element with the larger curve radius is increased in relation to the rotation speed of the ground engagement element with the smaller curve radius during cornering.
According to a further preferred embodiment, a first control model is defined, with which at least one limit for permitted slip of the ground engagement element of the vehicle at its ground contact point is determined, and a rotation speed value corresponding to the slip determined is calculated, a second control model being defined, with which the torque to said ground engagement element is determined as above on the basis of the friction against the ground, and the rotation speed of the rotary element being controlled according to the result of the second control model as long as the slip at said ground contact point determined with the first control model is on the permitted side of said limit.
There are therefore two different regulating principles via the first and second control model.
Furthermore, the first control model provides a limitation of permitted slip, and the function of the conventional differential lock is thus achieved. However, a certain rotation speed difference is permitted between the rotating elements which are regulated relative to one another with the first control model.
One of said control models is preferably selected on the basis of the prevailing operating conditions of the vehicle for controlling the rotation speed of the rotary element.
Selection of the control model is preferably carried out repeatedly, either continuously or intermittently, and automatically.
To be precise, the first control model for controlling the rotation speed of the rotary element is used when the second control model gives a calculated value of said rotation speed which means that the calculated slip according to the first control model lies outside said limit, the rotation speed of the rotary element being controlled so that it follows said limit.
Further preferred embodiment and advantages thereof emerge from the description below, the figures and the claims.